I. Field
The subject technology relates generally to communications systems and more particularly to systems and methods that transmit OFDM information by performing sub-carrier transform operations for transmitters over a higher bandwidth of transmitted samples.
II. Background
Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. The technology was first conceived during research into minimizing interference among channels near each other in frequency. In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Generally, priority is given to minimizing the interference, or crosstalk, among the channels and symbols comprising the data stream. Less importance is placed on perfecting individual channels.
In one area, OFDM has also been used in European digital audio broadcast services. The technology lends itself to digital television, and is being considered as a method of obtaining high-speed digital data transmission over conventional telephone lines. It is also used in wireless local area networks. Orthogonal Frequency Division Multiplexing can be considered an FDM modulation technique for transmitting large amounts of digital data over a radio wave where OFDM operates by splitting a radio signal into multiple smaller sub-signals or sub-carriers that are then transmitted simultaneously at different frequencies to the receiver. One advantage of OFDM technology is that it reduces the amount of crosstalk in signal transmissions where current specifications such as 802.11a WLAN, 802.16 and WiMAX technologies employ various OFDM aspects.
Traditional OFDM transmitters generally consist of an Inverse Fast Fourier Transform (IFFT) component followed by cyclic prefix addition. This operation is performed at the Nyquist rate at the transmitter which leads to very tight filtering requirements after digital to analog conversion (DAC) in order to remove images separated at the sampling frequency. Such filtering can be achieved in the analog or digital domain but the requirements for tight filtering cause several problems. One obvious problem is the cost associated with implementing a tight or steep filter. For instance, a first order filter would provide plus or minus 20 db per decade of filter roll-off near the corner frequency of the filter. Steeper or tighter 2nd or third order filters would provide 40 db or 60 db per decade roll-off respectively and are typically more complex to implement thus increasing filter cost. Implementing filtering in the digital domain leads to long filter lengths, increasing complexity. Also, filtering in the analog or digital domains reduces the resiliency of the transmitted OFDM signal to channel distortions.